In the past two decades x-ray lithography (XRL) has been developed as an alternative to optical lithography as feature sizes of silicon chips continue to shrink according to the Moore's law (S.P.I.E. Symp. Proceedings "Electron Beam, X-ray, and Ion-Beam Submicrometer Lithographies for Manufacturing", 1990-1996). Today, optical lithography is reaching some fundamental limits, and x-ray lithography is emerging as the primary successor technology needed for future lithography development. The two biggest challenges facing x-ray lithography are perhaps the fabrication of x-ray masks and the development of the x-ray sources. While the technology of mask patterning is evolving by means of electron beam lithography and advances in material science, economical exposure sources are still lacking. At present there are three types of x-ray sources that could provide sufficient flux for a reasonable exposure time (S.P.I.E. Symp. Proceedings "Electron Beam, X-ray, and Ion-Beam Submicrometer Lithographies for Manufacturing", 1990-1996): synchrotrons, plasma-based sources, and transition radiation (TR) sources. Among these, plasma-based sources are relatively easy to obtain but have the lowest available x-ray power level and no collimation. TR sources employ moderate energy electron linacs (25 MeV to 250 MeV) to bombard a stack of thin Beryllium (Be) foils. The resulting x-ray beam is well collimated but hollow in the forward direction. Special methods of making the cross section of the x-ray beam uniform and eliminating the background radiation are needed. Synchrotrons, on the other hand, are the preferred sources because they are powerful and stable. However, in order to generate x-rays having the optimal wavelength for XRL (around 1 nm), synchrotrons operate with high-energy electron beams (around 1 GeV when using conventional magnets and even 600 MeV when using superconducting dipoles). Hence the entire system (synchrotron, injector and radiation shielding) can be very expensive and complex. In addition, special beam lenses and steppers are necessary to facilitate wafer production, which translates to a huge initial investment for manufacturers.
There are a number of sources of x-rays which depend upon Compton Scattering. Compton scattering is a phenomenon of elastic scattering of photons and electrons. Since both the total energy and the momentum are conserved during the process, scattered photons with much higher energy (light with much shorter wavelength) can be obtained in this way. A laser beam collides nearly head-on with and is scattered off a high energy electron beam in order to generate x-rays (or even gamma-rays) for various applications. If .lambda..sub.L is the laser wavelength, .gamma. is the electron energy in units of its rest mass m.sub.e c.sup.2, and .theta. is the angle of the scattered x-ray photon with respect to the direction of the electron trajectory, then the wavelength of the scattered photon is ##EQU1##
These x-rays are naturally collimated around the forward direction of the electrons within an opening angle of 1/.gamma. due to the relativistic effect.
However, the total cross section of the Compton scattering is quite small, and is approximately given by the Thomson cross section when the energy of the scattered photon is much lower than the electron energy: ##EQU2## where r.sub.e =2.82.times.10.sup.-15 m is the classical electron radius. Suppose that N.sub.e number of electrons encounters N.sub.L number of photons with a common geometrical cross section A at an interaction rate f. Then the total number of scattered x-ray photons per unit time can be written as: ##EQU3##
From Eq. (3), we conclude that in order to obtain a significant x-ray flux, it is desirable to have both a high intensity photon beam and a high current electron beam along with a high interaction rate. Pat. Nos. 3,886,366; 4,598,415; 4,975,917; 5,247,562; 5,353,291; and 5,495,515, and the following articles: P. Sprangle et al., J. Appl. Phys. 72, 5032-5038 (1992), and J. Chen et al., Nucl. Instr. and Meth. in Phys. Res. A 341, 346-350 (1994), describe Compton scattered x-ray sources. The prior art apparatus for producing intense x-rays usually involve the use of a high-power, pulsed laser (either a solid-state laser with about 1 .mu.m wavelength or a gas CO2 laser with about 10 .mu.m wavelength) and an intense electron beam source from pulsed linacs, betatrons or storage rings. Lasers with short pulses are preferred because they can be focused to very small spots over the entire pulse length during the desired collision. Long pulse lasers or CW lasers such as in Pat. No. 5,247,562 when focused to a very small spot form a waiste-like shape so that the transverse area is large over much of the laser pulse length. In order for the laser pulse to be very small during the collision process, the laser pulse length (as well as the electron bunch length) must be smaller than, or on the order of, the depth of focus (or Rayleigh range) of the optical resonator. Some devices also employ a photon accumulating cavity such as a Fabry-Perot resonator (Pat. Nos. 4,598,415 and 5,495,575) or a ring cavity (Pat. No. 5,353,291) to build up the laser intensity.
Nevertheless, for applications in high throughput x-ray lithography, which require an average x-ray flux on the order of 10.sup.14 .about.10.sup.15 photons per second with their wavelengths around 1 nm, the repetition rate from a single-pass accelerator such as a linac of about 100 Hz is found to be too low, and hence the devices of Pat. No. 3,886,366 (an electrodynamics linac), Pat. No. 4,975,917 (a pulsed power linac), Pat. No. 5,353,291, and P. Sprangle et al., J. Appl. Phys. 72, 5032-5038 (1992) (an rf linac) are not satisfactory sources. On the other hand, a multiple-pass accelerator such as an electron storage ring, which does have a high enough repetition rate (more than 1 MHz), operates well at high energies but suffers beam instabilities at lower energies. Since the optimal wavelength of XRL is a factor of a thousand or ten thousands times shorter than the laser wavelengths, the energy of the electrons in the storage ring necessary to boost the laser photon energy by the same factor is 8 MeV (when using a solid-state laser) or 25 MeV (when using a CO2 laser). At either energy range, the number of electrons must be kept very low in order to avoid any beam instability. Thus, the x-ray yield is far from adequate, as in the cases of Pat. No. 4,598,415 and J. Chen et al., Nucl. Instr. and Meth. in Phys. Res. A 341, 346-350 (1994). The betatron of Pat. No. 5,353,291 has a high repetition rate but results in a continuous electron beam so that only a tiny fraction of the beam interacts with the desired short laser pulse. Thus, the x-ray yield is again far from adequate.
There are also proposals for employing a quasi-optical maser cavity to accumulate the electromagnetic radiation and/or to enhance the interaction rate for higher x-ray flux (Pat. No. 5,227,733). Both these methods use a millimeter wave source to scatter off a higher energy electron beam from a storage ring. However, millimeter wave sources have neither the power level nor the bunch compression schemes offered by solid-state lasers. Thus, they are not as favorable and convenient as using lasers for x-ray generation.